(a) Field of the Invention
This invention relates to a thermosetting resin composition for injection molding which is excellent in moldability under low injection pressure due to good flowability and thermal stability in a cylinder, and reactivity in a mold.
(b) Description of the Prior Art
In the injection molding of thermosetting resin, materials are desired to provide conflicting properties such as depressed reactivity in the cylinder, good flowability and rapid hardening in the mold. Many methods have been proposed to date for improving these properties.
As to phenolic resin compositions additives such as furfural and styrene have been incorporated in order to improve flowability and thermal stability in the cylinder. Their effects, however, are insufficient to enhance flowability. On the other hand, when the amount of addition is increased, these additives are disadvantageous in that hardening in the mold cannot proceed completely and defects are caused on the appearance and properties of molded articles
When curing accelerators are added to the above mentioned system, it is also unfavorable that only a small increase in curing rate is obtained in the mold and thermal stability of the system in the cylinder is remarkably damaged.
In addition, Japanese Laid Open Patent No. 18861/1979 discloses a method for incorporating inorganic salt hydrates and conducting cure acceleration by utilizing water generated by the decomposition of these hydrates in the mold. Japanese Laid-Open Patent No. 137294/1978 discloses a method for using usual pre-cured phenolic resin compositions in admixture with at least two ingredients selected from phenolic resin, hardener and fluidity modifier. Any of these methods can not provide satisfactory effects and have not yet been practically employed.
The required properties for these materials are such that flowability is maintained by inhibiting curing reaction at the material temperature of, for example, about 110.degree. C in the cylinder, whereas the curing reaction is not depressed at the material temperature of about 170.degree. C in the mold. Previous proposals have been unsatisfactory in such viewpoint.